Tryit Editor v2.9

Edit This Code:
<!DOCTYPE html>
<html>
<body>

<p>Click the button to return the index of the first element in the array that has a value above a specific number.</p>
<p>Minimum age: <input type="number" id="ageToCheck" value="18"></p>
<button onclick="myFunction()">Try it</button>

<p>Index: <span id="demo"></span></p>

<p><strong>Note:</strong> The findIndex() method is not supported in IE 11 (and earlier versions).</p>

<script>
var ages = [4, 12, 16, 20];

function checkAdult(age) {
    return age >= document.getElementById("ageToCheck").value;
}

function myFunction() {
    document.getElementById("demo").innerHTML = ages.findIndex(checkAdult);
}
</script>

</body>
</html>